2,617 research outputs found
The Proton Electric Pygmy Dipole Resonance
The evolution of the low-lying E1 strength in proton-rich nuclei is analyzed
in the framework of the self-consistent relativistic Hartree-Bogoliubov (RHB)
model and the relativistic quasiparticle random-phase approximation (RQRPA).
Model calculations are performed for a series of N=20 isotones and Z=18
isotopes. For nuclei close to the proton drip-line, the occurrence of
pronounced dipole peaks is predicted in the low-energy region below 10 MeV
excitation energy. From the analysis of the proton and neutron transition
densities and the structure of the RQRPA amplitudes, it is shown that these
states correspond to the proton pygmy dipole resonance.Comment: 7 pages, 4 figures, to be published in Phys. Rev. Let
Relativistic description of exotic collective excitation phenomena in atomic nuclei
The low-lying dipole and quadrupole states in neutron rich nuclei, are
studied within the fully self-consistent relativistic quasiparticle
random-phase approximation (RQRPA), formulated in the canonical basis of the
Relativistic Hartree-Bogoliubov model (RHB), which is extended to include the
density dependent interactions. In heavier nuclei, the low-lying E1 excited
state is identified as a pygmy dipole resonance (PDR), i.e. as a collective
mode of excess neutrons oscillating against a proton-neutron core. Isotopic
dependence of the PDR is characterized by a crossing between the PDR and
one-neutron separation energies. Already at moderate proton-neutron asymmetry
the PDR peak is calculated above the neutron emission threshold, indicating
important implications for the observation of the PDR in (gamma,gamma')
scattering, and on the theoretical predictions of the radiative neutron capture
rates in neutron-rich nuclei. In addition, a novel method is suggested for
determining the neutron skin of nuclei, based on measurement of excitation
energies of the Gamow-Teller resonance relative to the isobaric analog state.Comment: 8 pages, 3 figures, invited talk at the international workshop
"Blueprints for the nucleus: From First Principles to Collective Motion", May
17-22. 2004, Istanbul, Turkey; to appear in Int. J. Mod. Phys.
Calculation of stellar electron-capture cross sections on nuclei based on microscopic Skyrme functionals
A fully self-consistent microscopic framework for evaluation of nuclear
weak-interaction rates at finite temperature is introduced, based on Skyrme
functionals. The single-nucleon basis and the corresponding thermal occupation
factors of the initial nuclear state are determined in the finite-temperature
Skyrme Hartree-Fock model, and charge-exchange transitions to excited states
are computed using the finite-temperature RPA. Effective interactions are
implemented self-consistently: both the finite-temperature single-nucleon
Hartree-Fock equations and the matrix equations of RPA are based on the same
Skyrme energy density functional. Using a representative set of Skyrme
functionals, the model is applied in the calculation of stellar
electron-capture cross sections for selected nuclei in the iron mass group and
for neutron-rich Ge isotopes.Comment: 31 pages, 13 figures, submitted to Physical Review
Neutral-current neutrino-nucleus cross sections based on relativistic nuclear energy density functional
Background: Inelastic neutrino-nucleus scattering through the weak
neutral-current plays important role in stellar environment where transport of
neutrinos determine the rate of cooling. Since there are no direct experimental
data on neutral-current neutrino-nucleus cross sections available, only the
modeling of these reactions provides the relevant input for supernova
simulations. Purpose: To establish fully self-consistent framework for
neutral-current neutrino-nucleus reactions based on relativistic nuclear energy
density functional. Methods: Neutrino-nucleus cross sections are calculated
using weak Hamiltonian and nuclear properties of initial and excited states are
obtained with relativistic Hartree-Bogoliubov model and relativistic
quasiparticle random phase approximation that is extended to include pion
contributions for unnatural parity transitions. Results: Inelastic
neutral-current neutrino-nucleus cross sections for 12C, 16O, 56Fe, 56Ni, and
even isotopes {92-100}Mo as well as respective cross sections averaged over
distribution of supernova neutrinos. Conclusions: The present study provides
insight into neutrino-nucleus scattering cross sections in the neutral channel,
their theoretical uncertainty in view of recently developed microscopic models,
and paves the way for systematic self-consistent large-scale calculations
involving open-shell target nuclei.Comment: 25 pages, 9 figures, 2 tables, submitted to Physical Review
Toroidal dipole resonances in the relativistic random phase approximation
The isoscalar toroidal dipole strength distributions in spherical nuclei are
calculated in the framework of a fully consistent relativistic random phase
approximation. It is suggested that the recently observed "low-lying component
of the isoscalar dipole mode" might in fact correspond to the toroidal giant
dipole resonance. Although predicted by several theoretical models, the
existence of toroidal resonances has not yet been confirmed in experiment. The
strong mixing between the toroidal resonance and the dipole compression mode
might help to explain the large discrepancy between theory and experiment on
the position of isoscalar giant dipole resonances.Comment: 10 pages, 3 figures; Phys.Rev.C, in prin
Exotic modes of excitation in atomic nuclei far from stability
We review recent studies of the evolution of collective excitations in atomic
nuclei far from the valley of -stability. Collective degrees of freedom
govern essential aspects of nuclear structure, and for several decades the
study of collective modes such as rotations and vibrations has played a vital
role in our understanding of complex properties of nuclei. The multipole
response of unstable nuclei and the possible occurrence of new exotic modes of
excitation in weakly-bound nuclear systems, present a rapidly growing field of
research, but only few experimental studies of these phenomena have been
reported so far. Valuable data on the evolution of the low-energy dipole
response in unstable neutron-rich nuclei have been gathered in recent
experiments, but the available information is not sufficient to determine the
nature of observed excitations. Even in stable nuclei various modes of giant
collective oscillations had been predicted by theory years before they were
observed, and for that reason it is very important to perform detailed
theoretical studies of the evolution of collective modes of excitation in
nuclei far from stability. We therefore discuss the modern theoretical tools
that have been developed in recent years for the description of collective
excitations in weakly-bound nuclei. The review focuses on the applications of
these models to studies of the evolution of low-energy dipole modes from stable
nuclei to systems near the particle emission threshold, to analyses of various
isoscalar modes, those for which data are already available, as well as those
that could be observed in future experiments, to a description of
charge-exchange modes and their evolution in neutron-rich nuclei, and to
studies of the role of exotic low-energy modes in astrophysical processes.Comment: 123 pages, 59 figures, submitted to Reports on Progress in Physic
Pygmy dipole resonances in relativistic random phase approximation
The isovector dipole response in Pb is described in the framework of
a fully self-consistent relativistic random phase approximation. The NL3
parameter set for the effective mean-field Lagrangian with nonlinear meson
self-interaction terms, used in the present calculations, reproduces ground
state properties as well as the excitation energies of giant resonances in
nuclei. In addition to the isovector dipole resonance in Pb, the
present analysis predicts the occurrence of low-lying E1 peaks in the energy
region between 7 and 11 MeV. In particular, a collective state has been
identified whose dynamics correspond to that of a dipole pygmy resonance: the
vibration of the excess neutrons against the inert core composed of equal
number of protons and neutrons.Comment: LaTex 7 pages, 4 eps Figs, submitted to Phys. Lett.
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